The present invention relates to a device for protecting a digital signal transmission system in the case of failure affecting items of equipment that make up part of this system.
The invention particularly applies to a system for transmitting digital signals between an item of interrogating equipment and "n" items of responding terminal equipment in which at least the responses to such interrogations are carried over a transmission support that is common to the "n" items of responding terminal equipment.
Such a configuration is for example encountered where digital signals are transmitted between an interrogating item of terminal equipment and "n" responding items of terminal equipment that take the form of circuit boards grouped together in the same chassis or sub-assembly in order to constitute a complex functional assembly such as a digital signal multiplexing-demultiplexing unit, such transmission being typically employed for the purposes of centralizing, using an interrogating equipment board, alarm data relating to certain functional failures in the responding equipment boards.
Such equipment boards include logic circuits requiring to be powered by a source of direct current in order to operate, and a fuse is generally provided on each board, the fuse being designed to cut off the power supply when such boards are exposed to a risk of a deterioration as a result, for example, of excess voltage or excess current being present. The failure condition that was discussed above corresponds to this cutting off of the power supply.
Depending of the technology employed in the logic circuits used in such equipment, a problem can now occur. In effect, when such logic circuits are implemented in certain technologies, such as the HCMOS technology, which it is particularly useful to employ by virtue of the low power consumption of such circuits and their moderate cost, they can, in the absence of actual power to their terminals, continue to be powered by the digital signals that are applied thereto for processing, this latch-up type phenomenon being referred to by the name of "self-powering".
In a configuration such as the one described above that includes a common transmission support, this self-powering phenomenon can lead to serious disturbances which can even go as far as deterioration of the circuits. In effect, there is then the danger of having simultaneous responses on the common transmission support from several responding items of equipment, either of direct origin because of a failure involving certain ones of them or, indirectly as a result of a failure that involves the interrogating item of equipment which may then simultaneously interrogate several items of responding equipment.